For early detection of crimes, dangerous actions and the like at public places such as a railway station, an airport, a public square and a hall where an unspecified large number of people gather, there is a demand for technologies for monitoring a crowd with a camera and automatically detecting an abnormal state of the crowd. Examples of conventional techniques for detecting an abnormal state of the crowd include ones described below. In the following description, the term “crowd” refers to human beings when a crowd is monitored.
In a method and a program in Patent document 1, an observation area where particles move randomly is divided into small regions, and a region which shows a greater temporal change in the number of particles than the rest of the regions is detected as an abnormal region in which more particles make different movements than in the rest of the regions. The method and the program of Patent document 1 are also intended to be used for detecting a person or people in a crowd whose movements differ from the ones of other people.
Patent document 2 discloses a technique to calculate optical flows from video-image data and calculate an average of lengths (velocities) of the optical flows and a variance thereof, and detecting a local change in a crowd based on the average and the variance.